The present invention relates in general to dental matrices for engaging around a tooth, and in particular to a new and useful dental matrix made primarily of plastic material, requiring no external retainer for retaining the matrix on a tooth, and being adjustable for both increasing and decreasing the initial diameter of the matrix so that it can be used with teeth having different diameters.
A retainerless steel dental matrix was introduced by the present inventor in his U.S. Pat. No. 3,921,299 entitled RETAINERLESS DENTAL MATRIX AND METHOD OF MANUFACTURE, issued Nov. 25, 1975 and hereinafter referred to as the -299 patent. The -299 patent discloses the use of a lock loop for engaging around the steel matrix which includes undulations or ripples referred to as "impregnators" in the patent. FIG. 7 of the present application shows a lock loop 6 before it has been engaged onto a matrix and carrying the impregnators 7.
Prior to tightening the steel matrix on the tooth, the lock loop fits loosely over the matrix. The metal matrix is adjustable prior to being seated on the tooth. Tightening of the matrix is achieved by winding a coil which is provided on the matrix using a winding tool such as that disclosed by the inventor's U.S. Pat. No. 3,852,884 entitled WINDING AND TIGHTENING TOOL AND METHOD OF MANUFACTURING SAME, issued Dec. 10, 1974 and hereinafter referred to as the -884 patent.
Winding of the coil which in turn causes tightening of the band also causes metal of the band adjacent to the coil to rise up and be impaled on the sharp edges of the impregnators on the inside of the lock loop. This causes a locking of the matrix in its tightened position. The impregnators actually work harden the metal of the matrix which securely locks the matrix in position.
The present inventor has also introduced a plastic matrix which is disclosed in his U.S. Pat. No. 4,523,909 entitled PLASTIC DENTAL MATRIX AND METHOD OF MANUFACTURING SAME, issued June 18, 1985 and hereinafter referred to as the -909 patent. To accomodate the lower winding torques which are appropriate for plastic bands, the inventor also disclosed a dual winding tool in his U.S. Pat. No. 4,551,097 entitled DUAL-TORQUE WINDING AND TIGHTENING TOOL, issued Nov. 5, 1985, hereinafter referred to as the -097 patent.
The lock loop of the plastic matrix in the -909 patent included impregnators 7 as shown in FIG. 7. It has been found however that this provides an insufficient locking effect for the plastic matrix. While increasing the sharpness of the impregnators did increase the locking capacity somewhat, it also tended to tear the plastic matrix. Sharpened impregnators were thus found inappropriate for the plastic matrices. Notably impregnators do not work harden the plastic matrix of the -909 patent as they did the metal matrices.
An attempt was made to more securely clamp the lock loop onto the plastic matrix. While this did improve the ability of the lock loop to lock the matrix in position, it deprived the dentist of his ability to adjust the diameter of the matrix prior to seating it on the tooth. Without being able to adjust the starting diameter of the matrix, a larger supply of differently sized matrices would be necessary to accomodate the different sized teeth. The number of different sizes necessary would become impractically large. By making the plastic matrix adjustable in its initial diameter, far fewer different matrix sizes need be stocked.
It is important to note that even if an attempt is made to seat a matrix having only a slightly smaller diameter than a tooth, the matrix tends to collapse as the dentist tries to force the matrix down around the larger diameter tooth. The difference in diameter between the matrix and the tooth may even be so small as not to be apparent to the dentist until he actually tries to fit the matrix on the tooth. By making the matrix adjustable the dentist can provide a safety margin in making sure the matrix is at least bigger than the tooth before an attempt is made to fit the matrix on the tooth.
FIGS. 10 and 11 illustrate problems which may occur when utilizing the plastic matrix of the -909 patent.
As shown in FIG. 11, if the lock loop 6 is very firmly clamped onto the matrix for producing a secure locking of the matrix, and if the matrix is used on a tooth having too small a diameter for the matrix size, when a dentist attempts to roll the coil 4 using the winding tool, rather than pulling the excess portion 33 of the matrix 2 through the lock loop 6 as is required to adapt the matrix to the tooth, the coil tends to destructively "wrap under" at 36 the lock loop, without pulling the excess through the lock loop. The matrix thus fails to be tightened around the tooth.
FIG. 10 illustrates a technique which is sometimes used by a dentist to insure the correct tightening of a matrix 2 around an undersized tooth 5. The dentist utilizes his finger intra-orally to hold the matrix at the lock loop 6 while the coil 4 is wound. This then permits excess matrix portion 33 to be wound up on the coil and tightened on the tooth. This intra-oral manipulation is an awkward nuisance in the cluttered intra-oral area, and makes the use of the matrix more difficult.
FIG. 1 shows a plastic blank sheet member which is used to form the matrix 2 and which includes a substantially straight coiled portion with an end having a cross-hair or notch 3 for receiving a tool for forming the coil. The matrix also includes an intermediate curved tooth engaging portion that terminates at the receiving means in the form of an enlarged end that includes an extra laminated layer 24 with an edge 25 and a pair of opposed notches 8. This retaining means serves to receive and engage the lock loop of FIG. 7. FIG. 8 shows the lock loop 6 in place on the end of the matrix 2.
FIG. 16 illustrates another problem of the plastic matrix in patent -909. Since the impregnators 7 are near the middle of the matrix and the engagement of the lock loop 6 is rather tenuous at the top and bottom of the matrix, the matrix tends to wobble or pivot in the direction of the double arrows when the dentist inadvertently bumps against the outer most part of the lock loop, causing the lock loop to pivot. This pivoting has been found to cause the loosening of the coil and the matrix 2 from around the tooth 5.